Even if you're lucky, and it is git.internal - rather than source.internal with VSS on one and ClearCase on the other - there may be conflicting repository names. These'll be configured on developer's PCs, CI servers, automated deployment scripts. And that's just one domain name.
That's potentially a lot of work to connect two organizations: who may well not be connecting because of a merger - they may simply be contractors.
If ABC Corp and XZY Corp had their local stuff on internal.abc.com and internal.xyz.com, then all their old domains - and urls - keep working after they connect their networks. Whether and when to merge their services together becomes a decision based on the merits of each merge, rather than something that is forced because they choose a bad naming scheme.